1. Field of the Invention
The present invention relates to minimizing defects in components produced by lithography, particularly to the mitigation of substrate defects in reticles or masks utilized in extreme ultraviolet lithography, and more particularly to the use of a buffer layer deposited intermediate a reticle substrate and a reflective coating for mitigating substrate defects in reticles, the buffer layer being formed by a sequential coating and annealing process.
2. Description of Related Art
Extreme ultraviolet lithography (EUVL) systems are being developed for the production of electronic components formed on wafers via reflected radiation. The EUVL systems include reticles or masks that must be essentially free of defects that will print at the wafer which will yield defective components. The reticles, for example, may be fabricated by depositing highly reflective multilayer coatings, such as Mo/Si, on super polished substrates. Any localized structural imperfections on the reticle substrate may nucleate and evolve during the multilayer coating process into a defect that perturbs the reflected radiation field sufficiently to print at the water. Thus, there has been a need for mitigating the effect of small particle contaminants on the surface of the substrate that would nucleate a defect in the reflective coating.
The reticle defect problem may be divided into two components. First, there are the defects associated with the condition of the reticle substrate. These are particles, pits, or scratches on the reticle substrate that nucleate a growth defect in the multilayer coating. Second, there are the defects that are introduced during or after the multilayer coating process, which are particle contaminants that are embedded within or are sitting on the top surface of the coating. A low defect multilayer coating technology based on ion beam sputtering has been developed so that the coatings now being deposited are essentially defect-free, thus the greater risk is the starting conditions of the reticle substrate.
Modeling has been carried out that simulates the growth defects nucleated by spherical particles. The results have indicated that particles as small as about 25 nm in diameter will nucleate defects in multilayer coatings, which can image at the wafer. Hence, all particles of a size greater than about 25 nm must be removed from the reticle substrates prior to the deposition of the reflective coating. Currently, removal of particles from the reticle substrates is carried out by cleaning processes that are expected to be ineffective for the removal of particles of less than about 60 nm, particularly since verification that such small particles exist is difficult.
Prior efforts to resolve the reticle substrate defect problem involved a single layer buffer-layer, see K. B. Nguyen, et al., J. Vac. Sci. Technol. 11:2964 (1993), where a 200 nm thick single-layer amorphous silicon (a-Si) buffer-layer was deposited on the substrate prior to a Mo/Si multilayer deposition in an attempt to smooth out lithographically defined steps (defects) on a silicon surface. This resulted in some reduction in the defect height and the transition at the edges of the step was made less severe by the buffer-layer, which is advantageous. However, the surface roughness of the Si was increased significantly (from 0.2 nm to 0.7 nm), making this process impractical for EUV lithography.
Currently, there is a process having the capability of depositing a-Si approaching 200 nm in thickness with much lower roughness than that observed by Nguyen, referenced above. However, the large stress typically found in smooth, single-layer films like a-Si can limit the applicability of this single-layer approach for the buffer layer.
As an alternative to cleaning and single-layer buffer layers, the invention described and claimed in above referenced application Ser. No. 09/454,715, mitigates the effects of these small particles by depositing a multilayer film as a buffer layer in between the substrate and the reflective coating and the multilayer film can be deposited alone or with annealing during and or after deposition. The purpose of this buffer layer is to reduce the perturbation of the reflective coating due to particles, pits, or scratches on the substrate.
The present invention is an improvement over the technique of application Ser. No. 09/454,715 and involves mitigating the effect of the substrate particles on reticles by sequential coating and annealing. The sequential buffer layer coatings may be single layer or multilayer coatings and of the same or different materials, with either slow or fast post deposition annealing after each coating step.
The buffer layer coatings may be of the same material as the reflective deposited coating. The first buffer-layer may be tailored to flatten-out higher aspect ratio bumps and asperities. The second and subsequent buffer-layer coatings would be tailored to flatten out lower aspect ratio bumps and asperities. A reflective multilayer coating is later deposited on this healed surface to form a reticle blank for extreme ultraviolet (EVU) lithography.
It is an object of the present invention to mitigate the effects of substrate defects in coated reticles used for extreme ultraviolet lithography. A further object of the invention is to provide a reticle substrate with sequentially deposited and annealed buffer layers to mitigate substrate defects. A further object of the invention is to provide sequentially deposited buffer layer between a reticle substrate and a multilayer reflective coating with post annealing of each deposited buffer layer to mitigate the adverse effects of substrate defects. Another object of the invention is to provide a plurality of buffer layers between the substrate and a multilayer coating of a reticle utilized in extreme ultraviolet lithography, each buffer layer being annealed following deposition.
Yet another object of the invention involves the mitigation of substrate defects in reticles for extreme ultraviolet lithography using single layer or multilayer annealed buffer layers that are atomically smooth, have good smoothing properties, have low residual stress, and the multilayer buffer layers exhibit contraction during growth due to intermixing of the interfaces.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The present invention is directed to mitigate the effects of particles or other defects on a reticle substrate, particularly having a size  less than 60 nm, by depositing annealed single or multilayer films as buffer layers between the substrate and the reflective coatings. The purpose of this buffer layer is to reduce the perturbation of the reflective coatings due to the particles, pits, or scratches on the substrate. Specifically, the sequentially deposited and annealed buffer layers are designed to smooth out the substrate topography to a point where the remaining perturbations are too small to nucleate growth defects in the reflective coating that will print in an EUV lithography tool. The buffer layers are not used as a reflective coating, and thus their reflectivity to EUVL radiation need not be optimized.
The sequential buffer layers exhibit several basic characteristics. The buffer layers provide smoothing behavior, that is, relaxation of the surface height variations due to the particles, pits and scratches on the substrate. The multilayer buffer layers also exhibit volume contraction during growth due to intermixing at the interfaces of the alternating layers. The buffer layers do not substantially increase the high spatial frequency roughness of the substrate surface (e.g., roughness is typically less than about 0.3 nm rms). The buffer layers have sufficiently low residual stress (less than about 500 MPa) so that a total buffer layer thickness of up to about 100 xcexcm can be deposited without compromising the performance and stability of the reflective coating. Finally, the deposition process of each sequential buffer layer should be clean; there should must be substantially no defects added in the process of growing the buffer multilayer.
The buffer layers may be made from materials different from the reflective coating, or from the same materials. Although the buffer layers may be made from the same materials as the reflective multilayer, the buffer layers serve a different function and need not be optimized for high reflectance. For example the multilayer buffer layers may differ structurally from the reflective coating because of the intermixing at the layer interfaces.
Multilayer materials that are atomically smooth and have good smoothing properties may be used as the buffer layer. Multilayer systems that satisfy the above-listed multilayer buffer layer requirements include ion beam sputtered (IBS) molybdenum-silicon (Mo/Si), and MoRu/Be multilayer films. Other potential buffer layer multilayer materials that demonstrate the above-listed characteristics include Me/Si, Me/Be, Me/B, and Me/B4C, and where Me is a metal or alloy such as Mo, W, Ni, Cr, Ru, Rh, MoRu, or MoRh.
In the present invention, the buffer layers are deposited using an ultraclean ion beam sputtering process so that no defects are added in the deposition step. Each deposited buffer layer is annealed by a rapid or slow annealing technique. The buffer layers are deposited with minimal surface roughness and residual stress so that they do not compromise the performance of the reflective multilayer coating deposited on top of the buffer layers. The same ion beam tool can also be used to deposit the overlying reflective coating.
The present invention is a sequential coating and annealing technique to mitigate the effects of small particle contaminants in reticles for EUV lithography. The first step is to deposit a low particulate, very smooth buffer-layer coating which could have a significant amount of film stress; ion beam sputtered Mo/Si multilayer coatings and amorphous Si coatings are two examples. The first buffer-layer coating will likely be tailored to flatten out high aspect ratio bumps and asperities. Due to the significant film stress and the relatively thick substrates needed for the reticles, only a modest coating thickness can be applied at first. The sample is then annealed, either slowly or rapidly, with a temperature compatible with the substrates. Another buffer-layer coating layer is then applied and subsequently annealed, and the procedure is repeated as many times as is necessary to smooth the topology of the coating surface while retaining a surface with a sufficiently low roughness. The second and subsequent buffer-layer coatings could be tailored to flatten out low aspect ratio bumps and asperities. A reflective multilayer coating is later deposited on this healed surface to form a reticle blank for EUV lithography. In addition to Mo/Si, the multilayer coating may be composed of MoRu/Be or Mo/Be; and in addition to a-Si the single layer coatings may be composed of amorphous carbon. The annealing temperature and time would vary depending on the composition of the coating.
There is a strong commercial driving force for increased miniaturization in electronic devices, and EUVL systems have significant potential provided that a critical element thereof, the reflective multilayer reticle, is nearly defect-free.
Thus, the present invention has the potential to make such a system commercially viable.